Sigma-delta digital-to-analog converters (DACs) have gained wide popularity in integrated circuit design due to the high resolution and high linearity obtainable without the need for precise matching of on-chip components. The digital modulator shapes the quantization noise to be very low in the passband and much higher at out of band frequencies. The actual conversion to an analog signal is then performed by a very coarse DAC, which is typically implemented with a switched capacitor circuit, or a current-mode DAC. In the latter case, an array of current sources, either binarily-weighted or monotonic, is switched relative to the code coming out of the digital modulator.
An important function in the sigma-delta DAC is the analog filter at the output which removes the out-of-band quantization noise created by the digital noise shaper. The order of that filter is dependent on the out-of-band noise specification. One distinct advantage of the current-mode DAC implementation is that the filter can be a continuous-time filter. If a switched capacitor DAC is used, then the filter is typically switched-capacitor also. In the case of the continuous-time filter, the input signal, which is a current, must be converted to a voltage either before, or as a part of the filtering process.
If the current-to-voltage conversion is performed with an operational amplifier and a feedback resistor (i.e., transimpedance amplifier), then a virtual ground is presented to the current DAC. This is very desirable since modulation of the finite output impedance of the current sources in the DAC can result in distortion. Some single operational amplifier biquadratic filters (biquads) will accept a current input directly by eliminating an input resistor, which would save circuit area. However, the biquad input node is only a virtual ground for very low frequencies, and will have signal-dependent modulation at higher frequencies in the passband. This modulation can cause distortion for the above-mentioned reason. It would be desirable, then, to have a filter structure which is more area-efficient than one with two operational amplifiers and their associated resistors, which also does not cause signal-dependent modulation of the current input signal.